The invention relates to a method for obtaining oil products that can possibly be used either for the formulation of fuel for internal combustion engines with compression ignition (of the diesel type) or as a fuel, as well as the oil product of the gas oil type and the sub-products obtained by this method.
The gas oils currently found on the market in France in the form of fuels for diesel type engines, are products of the refining that contain sulfur in a quantity equal at most to 0.05% by weight (500 ppm, or parts per million); however, stricter and stricter standards of sulfur content are being considered, namely by the European Union for the year 2000 ( less than 350 ppm) and for 2005 (xe2x89xa650 ppm).
These gas oils are usually obtained following a treatment called hydrodesulfurization from very diverse hydrocarbon charges that can result from the direct distillation of a crude oil, from viscosity breaking, from hydroconversion or from catalytic cracking. Among the sulfur compounds contained in these charges, we find in order of increasing difficulty of elimination: thio alcohols, sulfides and disulfides, then thiophenes, and within this group, namely the family of dibenzenethiophenes (DBT) and their alkyl derivatives, which are known for being the compounds most resistant to desulfurization.
The catalytic hydrodesulfurization method consumes large quantities of hydrogen and must be operated at higher and higher temperatures and pressures or at a lower hourly space speed of the charge, and with more performing catalysts when we wish to eliminate said last quantities of sulfur contained in said organosulfur compounds; yet this makes the installations more and more costly and the method less interesting from an economic point of view.
This is why various alternatives have been proposed to try and correct these disadvantages.
This is namely the case of the method for obtaining a gas oil fraction from a hydrocarbon charge with a high sulfur content, described in the patent EP-0 621 334, which combines at least one liquid/liquid extraction step per solvent making it possible to extract at least in part the polyaromatic compounds that are contained in said charge and one or several hydrodesulfurization steps, more or less advanced. The major disadvantages of such a method are on the one hand that it combines one low pressure unit and one or several high pressure units and, on the other hand, that the extract obtained with a high concentration of sulfur or polyaromatic compound it is not easy to upgrade.
Another path that has been explored for some time, consists in using a method of desulfurization of hydrocarbon charges through the culture of specific microorganisms such as bacteria (or biocatalytic desulfurization), that produce enzymes that catalyze the degradation of the resistant compounds such as dibenzenethiophenes; however the difficulty of selecting said microorganisms came from their specific action by cutting the Cxe2x80x94C links of the organosulfur compounds, which generated a loss of quality of said charges and in particular of their calorific power; however genetically modified microorganisms, acting by oxidation cutting of the C-S links of the dibenzenethiophenes, were described namely in the patents U.S. Pat. No. 5,002,888, U.S. Pat. No. 5,104,801 and U.S. Pat. No. 5,132,219; the bio-desulfurization reaction passes through the following successive intermediary products: DBTsulfoxides, DBTsulfones; HPBSulfinates (hydroxyphenyl-benzenesulfinate), HPBSulfonates and the 2-hydroxybiphenyl and non organic sulfate final products.
In using specific genetically modified microorganisms as biocatalysts, we can stop the reaction at the stage of the intermediary products such as the hydroxyphenyl-benzenesulfinates or their derivatives, which are soluble in water and can thus be extracted from the hydrocarbon charges. However, the implementation of such a method requires the formation of an oil/water and biocatalyst emulsion, in a reactor whose ratio by volume can be 25/75%, and produces as a disadvantage the use of treatment capacities with high volumes.
U.S. Pat. No. 5,232,854 describes an advanced desulfurization method of a hydrocarbon charge, consisting of a traditional hydrodesulfurization step followed by a biocatalytic desulfurization step by incubation of the charge in the presence of oxygen with microorganisms such as those described in the above-mentioned patents and a step for separating the hydrocarbon effluent with a sulfur content of xe2x89xa6500 ppm from the residues in the form of non organic sulfurs. However, such a method does not make it possible to reduce the sulfur content of the charges treated at rates that are clearly less than 500 ppm, for example approximately 50 ppm.
This is why the object of this invention is thus to improve the advanced desulfurization of hydrocarbon charges with a high sulfur content, without implementing high temperature and pressure conditions, while upgrading the sub-products obtained.
Surprisingly, the Applicant has established that it is particularly wise to couple the extraction and oxidizing desulfurization steps that take place under low temperature and pressure conditions, close to room temperature and atmospheric pressure, and without consuming hydrogen, therefore very interesting from an economic point of view, and namely to concentrate the more resistant organosulfur compounds using an extraction method, then to treat the extracts obtained using a method of desulfurization by oxidation, so as to separate the hydrocarbon effluents obtained with a low sulfur content from the oxidized organosulfur compounds and recuperate said sub-products; the latter can then be upgraded, after treatment, namely as detergents.
With this end in view, the object of the invention is a method for obtaining oil products of the gas oil type with improved quality, from a gas oil fraction that contains organosulfur compounds of the dibenzenethiophene type and/or their derivatives, with initial and final boiling points usually ranging between 170 and 480xc2x0 C., characterized by the fact that it comprises at lest two steps, one a) a liquid/liquid extraction step wherein the gas oil fraction is put in contact with a solvent, so as to obtain a gas oil type raffinate with a low content of sulfur and aromatic compounds and an extract rich in solvent and a high content in sulfur an aromatic compounds and the other b) oxidation of the extract sulfur compounds, so as to obtain, after the separation, a heavy gas oil type hydrocarbon effluent with low sulfur content, and a residue comprising oxidized organosulfur compounds.
According to a first preferred mode of execution, the two steps are carried out at pressures that are less than or equal to 1 MPa, and at temperatures ranging between room temperature and 100xc2x0 C.
In particular, the solvent used in step a) is chosen from among the group consisting of methanol, acetonitrile, monomethyl formamide, dimethyl formamide, dimethyl acetamide, N-methyl-pyrrolidone, dimethyl sulfoxide and furfural.
According to a second preferred mode of execution, step b) consists of a biodesulfurization in which the extract is treated in a reactor, possibly cleared of the solvent, preferably at atmospheric pressure, in the presence of a biocatalyst in an aqueous phase, comprising an appropriate strain of microorganisms that produce enzymes capable of oxidizing the organosulfur compounds.
In particular, step b), when it is a biodesulfurization, is followed by a first step c) of separation of the oily phase that contains the hydrocarbon effluent and of the aqueous phase, where the latter contains the biocatalyst and the residue comprised of compounds of the organosulfinate type, in particular, benzenesulfinates and/or their derivatives; where the latter are formed by the opening of the thiophenic cycles by oxidation.
Preferably, the separation of the residue and the biocatalyst is done in during a second step d) of separation, that consists either of an extraction by a polar solvent or over ion exchanging resins, or of a filtration or of an adsorption over a polar solid support.
According to another variant of execution, step b) consists of a chemical oxidation; the latter can be executed namely with oxidizing agents of the peroxide type.
In this case, the residue of step b), obtained after separation, consists of compounds of the sulfoxide or sulfone type.
More specifically, the biodesulfurization step is carried out at pressures that are less than 1 MPa, and namely at atmospheric pressure, in the presence of air, and at temperatures that range between room temperature and 40xc2x0 C.
Advantageously, the beginning gas oil fraction is a direct distillation fraction, or comes from a hydrocracking, catalytic cracking, viscosity breaking or coking operation; its sulfur content is generally greater than or equal to 1% by weight.
Preferably, the hydrocarbon effluent of step b) has a final boiling point that is less than or equal to 380xc2x0 C.
Advantageously, the method as set forth in the invention comprises an additional distillation or decantation step of the extract from step a) making it possible to obtain an front product rich in solvent, which is recuperated and recycled during the extraction step and an end product depleted in solvent that is introduced in step b); one can also consider a distillation step of the raffinate from step a), in order to recuperate the low quantity of solvent it still contains.
According to a variant of execution, the method also consists of an additional hydrotreatment step of the hydrocarbon effluent obtained in step c), namely hydrodesulfurization under soft conditions.
In implementing the method of the invention, we can obtain an oil product of the gas oil type, with a sulfur content that is less than 500 ppm and preferably less than 50 ppm.
Compounds of the benzenesulfinate type obtained by the method as set forth in the invention, after having undergone an additional treatment such as oxidation followed by alkylation or alcylation, can be used as hydrotropic agents or surfactants, namely in the manufacturing of detergents.
The chemical oxidation of step b) can be carried out in a homogenous phase at low temperature (approximately 70-80xc2x0 C.) and at atmospheric pressure, in the presence of a peroxidizing agent (for example hydrogen peroxide) and an acid catalyst such as acetic acid or sulfuric acid. The separation of the sulfones obtained and the hydrocarbon effluent can be done on an adsorbent mass, such as for example silica gel. Any other type of oxidation can be used, namely an oxidation by Oxon (potassium peroxy-monosulfate) is also possible.
A first advantage presented by the method as set forth in the invention is to allow the use of hydrocarbon fractions, for example of the gas oil type, with an initial boiling point of approximately 170xc2x0 C. and a final boiling point of approximately 375xc2x0 C. (the temperatures of the boiling points as indicated are measured as according to the TBP xe2x80x9ctrue boiling pointxe2x80x9d0 distillation method), having already undergone a traditional catalytic hydrodesulfurization and with a residual sulfur content that is less than 500 ppm, and to concentrate, using the liquid-liquid type extraction step with a solvent that has an increased selectivity for dibenzenethiophenes and their derivatives, said compounds in the extract phase that is sent to the following step. The raffinate obtained in this extraction consists of a hydrocarbon charge with a low content of sulfur and aromatic compounds, that is sent to the engine gas oil pool (the pool is the name that designates all base products used to manufacture an oil product).
A second advantage is to carry out the two steps at a pressure that is less than 1 MPa and a temperature that is less than 100xc2x0 C., and preferably at atmospheric pressure and room temperature.
We can also consider previously subjecting the beginning hydrocarbon fraction, with a sulfur content that is less than 1% by weight, to a fractionation under conditions that make it possible to obtain a front product that is a fraction whose final boiling point is less than 320xc2x0 C. (considering the boiling point of the dibenzenethiophene is close to 330xc2x0 C.) with a sulfur content that is less than 1% by weight, and an end product whose initial boiling point is greater than 320xc2x0 C. (for example a TBP fraction of 320-380xc2x0 C.), comprising a high concentration of refractory sulfur compounds and that will be sent to the following extraction step.
As beginning gas oil fraction we can also use LCO type fractions (very aromatic xe2x80x9cLight cycle oilxe2x80x9d0 fraction produced by effluents of a catalytic cracking unit), with much higher sulfur contents that can reach 2.5% by weight, or even more.
The liquid-liquid extraction step is carried out in the traditional way, against the current, in an extraction column whose packing is, for example, fixed disks and rings, rotary disks or static mixers, preferably with 5 to 10 theoretical stages, at a temperature that ranges between 30xc2x0 C. and 100xc2x0 C., and under a pressure that ranges between 0.1 and 1 MPa; generally, the solvent to charge volume ratio is between 0.2/l and 5/l.
The solvent is preferably chosen from the group of solvents that make it possible to extract at least a part of the mono and polyaromatic and sulfur compounds, where this group is comprised of methanol, acetonitrile, monomethyl formamide, dimethyl formamide, dimethyl acetamide, N-methyl-pyrrolidone, dimethyl sulfoxide and furfural. We can also add a cosolvent that can be a linear or branched alcohol to the solvent.
We thus obtain a raffinate that is a gas oil fraction with a low sulfur content and that can be sent to the engine gas oil pool, after having been purified by eliminating the traces of solvent it contains, by distillation; furthermore, the extract obtained, with a high solvent content and high in sulfur and aromatic compounds, will be sent to step b), after having possibly been previously sent to an area of distillation or to any other separation device, in order to recuperate a fraction that contains almost all the solvent.
The oxidation step b) of the sulfur compounds, is carried out so as to obtain, after separation, a heavy gas oil type hydrocarbon effluent with a low sulfur content, that can be sent to the domestic fuel pool, and a residue that contains oxidized organosulfur compounds.
When step b) is a biodesulfurization, it was noticed that the efficiency of the desulfurization depended on the nature of the charge and that it decreased based on whether the gas oil was obtained by cocking, viscosity breaking, catalytic cracking (LCO) and by direct distillation (GOSR, xe2x80x9cGas Oil Straight Run).